It is known that fuel cells enable electrical energy to be produced directly via an electrochemical redox reaction using hydrogen (the fuel) and oxygen (the oxidant), without passing via a mechanical energy conversion step. This technology seems promising, especially for motor vehicle applications.
A fuel cell comprises in general the series combination of unitary elements each consisting essentially of an anode and a cathode separated by a polymer membrane allowing ions to pass from the anode to the cathode. This combination is also referred to as a stack. A fuel cell comprises an anode circuit, via which the fuel, for example hydrogen, is brought to the anode, and a cathode circuit, by which the oxidant, for example oxygen, is brought to the cathode, during the operating phases of the cell.
On the other hand, when the fuel cell is shut down, the cell's supply of fuel and oxidant is cut off. Yet it has been observed that the presence of fuel in the anode circuit of the cell, even during shutdown phases, makes it possible to slow down the degradation of the cell, and thus to lengthen the service life thereof. Indeed, the presence of hydrogen in the cell makes it possible to guarantee a zero electrochemical voltage, and thus to prevent unwanted electrochemical reactions during long periods when the cell remains shut down.
Fuel cells are known in which a presence of hydrogen at the anode during short shutdown phases is achieved by terminating the shutdown with an excess of hydrogen in the anode circuit. In this case, the cathode circuit is pressurized with nitrogen taken from the air. However, the known fuel cells do not make it possible to achieve perfect leaktightness of the stack. Indeed, the stack consists of bipolar plates and membrane electrode assemblies, the leaktightness between the various components being ensured with the aid of seals that have a certain gas permeability. Thus, a gas such as hydrogen or other fuel present at the anode has a tendency to migrate to the outside of the cell over time. Conversely, the air present in the atmosphere has a tendency to enter the anode, which proves prejudicial for the service life of the fuel cells. Indeed, the oxygen introduced with the air has a tendency to react with the hydrogen due to the catalyst present in the fuel cell, which tends to make the residual hydrogen disappear. When the hydrogen has completely disappeared, the oxygen then imposes its electrochemical potential of 1.48V on the electrodes, which promotes corrosion.
In order to overcome this drawback, a solution has been proposed that consists, when the hydrogen concentration passes below a certain threshold, in opening the tank in order to reintroduce a certain pressure of hydrogen into the stack. This solution, although effective, is not however satisfactory in terms of safety. Specifically, it would mean that the vehicle could open the main valve of the hydrogen tank without human presence, which proves to be dangerous.
Also known, from application WO2007/090284, are fuel cell systems comprising a main tank that delivers hydrogen during an operating phase of the fuel cell and a secondary tank that delivers hydrogen during the shutdown of the fuel cell. However, the system described in this application does not guarantee that a hydrogen presence is maintained until the next restarting.